1. Field of the Invention
The present invention relates to exhaust aftertreatment systems for use with combustion engines, and, more particularly, to exhaust aftertreatment systems for use with diesel engines.
2. Description of the Related Art
The control of nitrogen oxide (NOx) emissions from internal combustion engines and particulate matter (PM) emissions from compression (diesel) combustion engines is an environmental problem. Gasoline engine vehicles use three-way catalysts to control such emissions, because their exhaust gases lack oxygen. But so-called xe2x80x9clean-burnxe2x80x9d gas engines and diesel engines have enough oxygen in their exhausts that conventional catalytic systems are not effective. Lean-burn, high air-to-fuel ratio, engines are certain to become more important in meeting the mandated fuel economy requirements of next-generation vehicles. Fuel economy is improved since operating an engine stoichiometrically lean improves the combustion efficiency and power output. But excessive oxygen in lean-burn and diesel engine exhausts inhibit NOx removal in conventional three-way catalytic converters. An effective and durable catalyst for controlling NOx emissions under net oxidizing conditions is also critical for lean burn and diesel engines.
Exhaust emission control systems which have a NOx adsorber are known, in particular for cleaning exhaust gas from motor-vehicle combustion engines operated predominantly on a lean mixture. In lean operating phases of the combustion device emitting the exhaust gas to be cleaned, such as a motor-vehicle spark-ignition engine operated predominantly on a lean mixture or a diesel engine, the NOx adsorber stores NOx contained in the exhaust gas by adsorption. It is possible for the NOx not to be reduced adequately to nitrogen, for example by a three-way catalyst, because of the oxygen excess and consequently the lack of reducing agents in the exhaust gas. The loading of the NOx adsorber, also known as a nitrogen-oxide adsorber catalyst, with NOx, mainly in nitrate form, increases continuously in the course of a lean operating phase. When its storage capacity is exhausted and it cannot adsorb any further NOx, a changeover is made from the lean operation of the combustion device, which corresponds to an adsorption phase of the NOx adsorber, briefly to a rich operating phase, in which the NOx adsorber is fed an exhaust gas with an at most stoichiometric exhaust air ratio (generally with a substoichiometric air ratio, i.e., with a rich composition of the exhaust gas). This may take place, for example, by changing over the combustion device from the previous lean operation with an at least stoichiometric oxygen component in the fuel/air mixture to be burned to rich operation with a rich mixture; by injecting reducing agents directly into the exhaust gas upstream of the NOx adsorber; and/or by other methods. The rich operating phase corresponds to a regeneration phase of the NOx adsorber, in which the NOx temporarily stored in it are desorbed and then converted by reducing agents adequately present in the fed-in rich exhaust gas. The conversion may take place, for example, in the nitrogen-oxide adsorber body itself if a three-way catalyst is integrated therein; or in a downstream nitrogen-oxide reduction catalyst; or, for example, also by exhaust gas recirculation. The use of a three-way catalyst ensures effective NOx conversion even in the stoichiometric range of the combustion device.
Typical fuels for internal combustion engines contain sulfur. When such a fuel is burned, sulfur contained in the fuel is caused to burn and produce oxides of sulfur (SOx), such as SO2 and SO3, which become exhaust gas components. When the exhaust gas containing SOx reaches the catalyst, SOx is apt to be adsorbed onto the catalyst whereby stable inorganic sulfates are formed. Since the sulfates are more stable than nitrates, the sulfates are difficult to decompose and release, and tend to be accumulated in the catalyst. If the amount of SOx accumulated in the catalyst increases, the ability of the catalyst to adsorb and thus reduce other harmful components (HC, CO, NOx) in the exhaust gas may deteriorate. This is generally called xe2x80x9csulfur poisoningxe2x80x9d.
A particulate filter, also commonly used with diesel engines, is used to prevent carbon particles from soot or PM from exiting the tailpipe. Since the particulate filter has a limited storage capacity, it is periodically regenerated. In one approach, during the regeneration process, exhaust temperature is increased to ignite carbon particles stored in the particulate filter. By burning the stored carbon particles, the filter is regenerated and able to again store the carbon particles.
What is needed in the art is an exhaust aftertreatment system and method for a diesel engine which more effectively reduces NOx and PM.
The present invention provides a diesel engine exhaust aftertreatment system that reduces NOx and PM. The aftertreatment system may include one or more exhaust flow paths, or xe2x80x9clegsxe2x80x9d, and each leg of the exhaust system may contain any combination of an injector, NOx adsorber element(s), with an upstream or downstream diesel oxidation catalyst, and optional upstream or downstream particulate trap or other substrate. A single pass exhaust aftertreatment system may also contain an exhaust bypass. A multi-pass exhaust aftertreatment system includes multiple exhaust legs, whereby the individual flow paths can be partially or fully blocked by a valve while the exhaust continues to flow from the engine to atmosphere through the open flow path(s). A small amount of exhaust continues to flow through the leg that is blocked by the system valve, and only a small amount of auxiliary fuel is required to change the lean exhaust to rich, such that the NOx adsorber can be regenerated. The other leg(s) of the exhaust system continue to adsorb NOx during normal operation, enabling the total system to emit low NOx and PM during the entire engine""s operating regime.
The invention comprises, in one form thereof, an exhaust aftertreatment system for use with an internal combustion engine. A manifold has at least one inlet and a plurality of outlets. A plurality of legs are connected with a respective manifold outlet. Each leg has a NOx adsorber therein. At least one valve is positioned in association with at least one leg for at least partially opening and closing at least one leg.
The invention comprises, in another form thereof, an exhaust aftertreatment system for use with an internal combustion engine. At least one leg has a multi-stage NOx adsorber, with each NOx adsorber stage corresponding to a different temperature range of NOx adsorption.
An advantage of the present invention is that the NOx adsorber elements may be configured as replaceable elements.
Yet another advantage is that the NOx adsorber elements may be coated with a material to improve NOx adsorption over a broader temperature range.
Still another advantage is the exhaust aftertreatment system is relatively low cost in comparison with existing systems.
A still further advantage is that the exhaust aftertreatment system may be provided with a low cost fuel injector.
Another advantage is that in a multi-pass system, exhaust flow control valves and exhaust aftertreatment regeneration strategies are provided.
Yet another advantage is that the exhaust aftertreatment system is designed for compact installation on a mobile vehicle platform, but may also be installed on stationary engine applications, such as a generator.